The hepatic microsomal hemoproteins cytochromes P450 (P450) include multiple constitutive and inducible enzymes. These monomeric hemoproteins (MW @ 50kDa) contain one prosthetic heme (iron-protoporphyrin IX) moiety/mole of enzyme. In spite of their identical hememoieties, P450s differ functionally, a property conferred by individual heme-apocytochrome microenvironments. P450s are instrumental in the oxidative/reductive metabolism of various physiologically relevant endobiotics and xenobiotics. However, although all these reactions result in the formation of readily excretable products, not all are beneficial. P450s catalyze the metabolism of some substrates to radicals and other reactive species that can induce toxicity/pathological damage. Furthermore, in the course of certain redox reactions, the participating P450 is sacrificed in a process classified as a mechanism-based or "suicide" inactivation. To date, three distinct mechanisms of substrate-mediated P450 inactivation have beencharacterized: (a) prosthetic heme destruction via N-alkyl/arylation [i.e., allylisopropylacetamide (AIA), secobarbita (SB)]; (b) apocytochrome alkylatin by a reactive intermediate (chloramphenicol, SB, 11-undecynoic acid); and (c) destruction of the prosthetic hemeto products that irreversibly bind to the apocytochrome [CCl4,spironolactone (SPL), 3,5-dicarbethoxy-2,6-dimethyl-4-ethyl-1,4-dihydropyridine (DDEP)]. By definition, "suicide" inactivations occur at the active site. Isolation and structural characterization of the N-alkylated hemehas unequivocally established this criterion for N-alkylation of P450 heme. However, the criterion for "suicide inactivation" has not been rigorously applied to modes b or c of drug-induced P450 destruction because the inaccessibility to structural analyses of the highly hydrophobic apoP450 active site regions and their resistance to proteolytic digestion (with an array of proteases) have until now largely precluded their definitive mechanistic classification. Using lysyl endopeptidase C and/or pepsin digestion as well as CNBr cleavage, the P450 peptides modified by either the heme or the drug have been HPLC mapped and isolated as a first stepto their identification and structural characterization by mass spectrometric analyses using an array of mass spectrometric techniques provided by the Mass Spectrometry Facility. To date, two different P450 peptides alkylated by heme and SB have been isolated and characterized, using ESMS and MSLDI-MS. Such structural characterization will greatly contribute to the definitive mechanistic elucidation of modes b and c inactivation processes.